On Sunday, the king waved to onlookers as he walked through the rain to the church on the Sandringham Estate in Norfolk.
Prince Charles, 75, appeared to be in good spirits as he made the short trip to St Mary Magdalene Church, accompanied by the Queen, 76, and the Reverend Paul Williams.
Despite the drizzling rain, the three people were seen cheerfully holding umbrellas.
It is the monarch’s first public appearance since the Duke of Sussex suggested in an interview with a US breakfast TV show that his father’s illness could lead to reconciliation between the two countries.
The King is recuperating at his home in Norfolk after starting cancer treatment.
He has made several public appearances since announcing his diagnosis in February, including being photographed inside a Bentley limousine after a brief meeting with the Duke of Sussex.
The duke told Good Morning America on Friday that he loves his family and was “grateful” to be able to spend time with his father during his short trip.
The TV interview was filmed in Whistler, Canada, for an event promoting the Invictus Games, and came 10 days after Prince Harry’s 30-minute meeting with the King.
The interview, which aired on the ABC, suggested that a family illness could have a “unifying effect”, with the duke asked: “Is that possible in this case?” He replied, “Yeah, sure.”